High-pressure metallic vapor discharge lamp including mercury and thallium iodide



March 4, 1969 D. A. LARSON 3,431,447

HIGH-PRESSURE, METALLIC VAPOR DISCHARGE LAMP INCLUDING MERCURY AND THALLIUM 10mm: Filed Feb. 16, 1966 WITNESSES INVENTOR m Daniel A. Larson X4; XJW WW ATTORNEY United States Patent "ice 3,431,447 HIGH-PRESSURE METALLIC VAPOR DISCHARGE LAMP INCLUDING MERCURY AND THALLIUM IODIDE Daniel A. Larson, Cedar Grove, N.J., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 16, 1966, Ser. No. 527,676 US. Cl. 31325 Claims Int. Cl. H01 1/02, 7/24, 61/52 ABSTRACT OF THE DIS CLOSURE A high efliciency high-pressure, mercury-vapor discharge lamp oharacterized by the production of a relatively wide band of energy at about 5350 angstroms and an output in excess of 100 lumens per watt when operated with a predetermined power input and which includes as a discharge sustaining filling within said are tube an amount of mercury of from about 5.0 to 8.0 milligrams per cubic centimeter of arc tube volume and an amount of thallium iodide in excess of 0.5 milligram per cubic centimeter of arc tube volume and preferably about 2.3 milligrams per cubic centimeter of arc tube volume.

This invention relates to discharge devices and more particularly to a high-pressure, mercury-vapor, discharge lamp of the additive type with increased lumen output.

High pressure, mercury vapor, discharge lamps HPMV lamps) are well known and have been extensively used for industrial and highway lighting applications, as well as for numerous other purposes. The use of additive metal halides in mercury vapor lamps is now well known. Such halide additives have been used to increase efficiencies, to supplement and convert spectral emissions of the mercury vapor to improve the color rendition of illuminated objects and to provide lighting of a particular color. With respect to the latter, a pure mercury discharge concentrates radiations primarily in the geen and yellow portions of the spectrum while with the addition of selected quantities of, for example, lithium iodide, strong red radiations and a red glow is apparent. With indium tri-iodide, the discharge is very blue. The addition of selected quantities of sodium iodide will produce a light that has a yellow-orange color. Color correction as well as control have also been accomplished by the use of phosphor coatings on the outer bulb but such coatings require an increased source size and present optical problems in the efiicient utilization of the light.

For a number of years, considerable eifort has been expended by the lamp industry to improve the efficiencies of mercury discharge lamps. The luminous efiiciency of a high-pressure, mercury-vapor discharge lamp is approximately 54 lumens per watt. By utilizing various metal and metal iodide additives, the luminous efiiciency of the high-pressure, mercury-vapor discharge lamp, with additives, has been increased in recent years to from 80 to 90 lumens per watt. However, even with the advent of the additive type lamp along with improved structural design, outputs of approximately 90 lumens per watt appear to be the maximum which is consistently reproducible.

It is an object of the present invention to provide an improved high-pressure gas-discharge lamp of the additive type which provides improved output and improved efiiciency in the production of visible light.

Another object of this invention is to provide a highpressure gas-discharge lamp of the additive type which provides maximum intensity in the area of maximum eye sensitivity.

Patented Mar. 4, 1969 A further object of the present invention is to provide a high-pressure gas-discharge lamp of the additive type in which an increased amount of additive is vaporized.

A still further object of this invention is the provision of a mercury vapor lamp having a luminous efliciency in excess of lumens per watt.

Briefly, the foregoing objects are accomplished by substantially increasing the amount of mercury in the lamp to broaden the band of radiation in the area of 5350 angstroms to reduce absorption while increasing the amount of additive vaporized by increasing the temperature at the normally cold ends of the lamp through a reduction in the distance between the are producing ends of the electrodes of the lamp and the normally coldest point in said lamp.

The foregoing objects as well as many of the attendant advantages of the present invention will become better understood as the detailed description which follows is considered in conjunction with the sole figure of the accompanying drawing which illustrates a lamp constructed in accordance with the present invention with the energizing power source electrically connected thereto.

While the teachings of the present invention are applicable to lamps and discharge devices of varying size and designed wattage input, high-pressure, gas-discharge lamps having a designed power input of 400 watts are used extensively, and hence such a lamp has been i1- lustrated and will be described.

A specific form of the invention illustrated in the drawing is generally similar in construction to the usual high pressure mercury vapor (HPMV) lamp, such as that described in US. Patent No. 2,748,303, dated May 29, 1956, to Thorington. The lamp, generally designated 10 includes a radiation transmitting sealed outer envelope 12 spaced from and surrounding an inner envelope or arc tube 14. The inner envelope 14 is conventionally mounted within outer envelope 12 by means of a supporting frame 16 carried by one of the two lead-in conductors 18, each of which is sealed through the outer envelope 12 by a conventional re-entrant stem press 20 conected to a standard Mogul base 22. Mogul base 22 facilitates electrical connection to a power source 24 in a well known manner.

Sealed within the inner envelope 14 and disposed at oposite ends thereof are a pair of tungsten operating electrodes 26. Adjacent one of the operating electrodes 26 is a tungsten starting electrode 28. Each of the electrodes 26, 28 are electrically connected to lead-in conductors 18. A starting resistor 30 is connected between the starting electrode 28 and one of lead-in conductors 22. Ribbon conductors 32 are employed to facilitate the hermetic sealing of electrodes 26, 28 through the ends of the inner envelope 14.

In the standard 400 Watt mercury lamp, itis conventional to use operating electrodes having inwardly extending tips situated approximately 10 millimeters from the junction of the metal and the quartz tube as indicated at 27. Since the vapor pressure of the metal iodide additive is a function of the temperature at the coldest point on the inner surface of the arc tube and the coldest point generally occurs behind the electrode, it has been found that the temperature at the ends of the arc tube behind the electrodes can be increased by shortening the electrode. Since the heating of the end of the arc tube chamher is due in a large extent to the conduction of heat from the high temperature plasma of the are through the adjacent gas to the quartz wall or end chamber, it has been found that by limiting the distance between the tip of the electrode and the junction of the metal and quartz at 27 to not more than 6 millimeters, that an increase in efliciency of approximately 15% will accrue. This increase in etficiency results from the higher metal iodide vapor pressure and the longer arc length provided by the decrease in electrode length.

Within inner envelope or are tube 14 is disposed a predetermined amount of mercury 34, and a small charge of inert ionizable starting gas, such as argon. In addition, a predetermined amount of thallium iodide 36 is sealed within inner envelope 14. In the conventional 400 watt mercury discharge lamp, it is common practice to use 66 milligrams of mercury in a 22 cubic centimeter arc tube and the addition of larger quantities of mercury in a conventional lamp provides little if any increase in lumen output.

In the lamp of the present invention employing a standard 400 watt quartz arc tube 14 having a 22 millimeter outside diameter and about 22 cubic centimeters of volume, it has been found that by doubling the mercury dose to 132 milligrams and employing a 50 milligram charge of thallium iodide, an unexpected additional increase in efficiency of approximately will be noted. The combination of the shortening of the electrodes, coupled with a 100% increase in the mercury charge and an increase in the thallium iodide charge of 25 milligrams over that which is normally vaporized produces an increase in lumen output from 32x10 lumens for conventional additive lamps to 43x10 lumens which represents an efliciency increase from a conventional 80 lumens per watt to 107 lumens per watt with the lamp of the present invention. There is also an increase of approximately 50 C. in minimum arc tube temperature with a minimum temperature of 650 C. being maintained.

Although, from the standpoint of material conservation and reasonable voltage utilization, charges of 132 milligrams of mercury and 50 milligrams of thallium iodide are preferred, substantial increases in efficiency are available in the thallium iodide additive lamp of the present invention when a mercury charge of from 110 to 176 milligrams is employed with charges of thallium iodide in excess of 11 milligrams. Mercury charges between 66 and 110 milligrams, although producing slight increases in etficiency, are not considered measurably significant while charges in excess of 176 milligrams require excessive operating voltages of over 300 volts, require high cool spot temperatures to produce total vaporization of the mercury and produce operating pressures in excess of the safe limits of the cylindrical quartz arc tube. Accordingly, mercury charges in excess of 176 milligrams are therefore considered impractical for a gas discharge lamp of the type contemplated by the present invention.

The foregoing charges are, of course, designated for a 22 cubic centimeter arc tube of the type previously described and therefore represent charges of mercury of from 5.0 to 8.0 mg./cc. and a thallium iodide charge in excess of 0.5 mg./cc. in arc tubes enclosing different volumes.

Additional metal halides such as, for example, indium iodide, sodium iodide, bismuth iodide or lithium iodide may, of course, be added to the charge in minute quantities to provide changes in the color quality of the radiated light without departing from the scope of the present invention. In most instances, however, a slight reduction in efficiency will occur and for purposes of maximum efficiency and stability their use is not preferred.

In operation, the high-pressure, mercury-vapor discharge lamp employing a 22 cubic centimeter arc tube and a. charge of approximately 2.3 mg./cc. of arc tube volume of thallium iodide and approximately 6.0 mg./cc. of arc tube volume of mercury along with a small charge of argon, is initially started. A discharge will be established between the tungsten starting electrode 28 and the adjacent operating electrode 26 through the argon or other starting gas. Thereafter, a discharge will be established between the two operating electrodes 26 which will heat all the mercury charge 34 to a fully vaporized condition.

As the mercury becomes fully vaporized, the operating temperature of the arc tube 14 will increase to a point at which the coolest point on the inner wall of the arc tube is in excess of 650 C., at which time a substantial portion of the thallium iodide will be vaporized.

While the lamp of the present invention is less pleasing as a light source than many mercury additive lamps insofar as color rendition of illuminated objects is concerned, the increased mercury pressure and increased voltage broadens the thallium output in the area of 5350 angstroms. This substantially monochromatic light source produces a band approximately angstroms in width of high intensity energy at 5350 angstroms which, being near the point of maximum eye sensitivity, provides a high intensity green light source. This green color and high lumen output has particular application for the lighting of parks and athletic fields where the green light will supplement and complement the natural greens of the illuminated area. Other interesting applications for this wide band, high energy output in the 5350 angstrorn area is its use in the field of photocopying and also as an excitation source for ruby lasers.

As can be seen from the foregoing, the improved high pressure gas discharge lamp of the present invention provides a highly eflicient, comparatively monochromatic light source producing a lumen output substantially in excess of that which can normally be achieved with a mercury additive lamp.

While the preferred embodiment of the present invention has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim as my invention:

1. A high efficiency discharge lamp comprising:

(a) a radiation transmitting elongated arc tube enclosing a predetermined volume;

(b) arc supporting electrodes disposed within said are tube adjacent the ends thereof;

(0) lead-in conductors connected to said electrodes and sealed through said arc tube; and

(d) a discharge sustaining filling within said are tube comprising mercury in an amount of from about 5.0 to 8.0 milligrams per cubic centimeter of arc tube volume which when fully vaporized during normal operation of said lamp will provide a predetermined mercury vapor pressure in said are tube, and thallium iodide in an amount of at least approximately 0.5 milligram per cubic centimeter of volume within said are tube, wherein upon full vaporization of said mercury and substantial vaporization of said thallium iodide a relatively wide band of energy at about 5350 angstroms is produced.

2. A discharge lamp according to claim 1 wherein said mercury from about 5.0 to 8.0 milligrams per cubic centimeter is approximately 6.0 milligrams per cubic centimeter of arc tube volume.

3. A discharge lamp according to claim 1 wherein the distance between the ends of said are tube and the remote end of its adjacent are supporting electrode is minimized.

4. A discharge lamp according to claim 1 wherein the minimum temperature on said are tube is at least 650 C.

5. A discharge lamp according to claim 2 wherein said thallium iodide is approximately 2.3 milligrams per cubic centimeter of arc tube volume.

6. A highly efficient discharge lamp adapted to be operated with apredetermined power input and comprising:

(a) a radiation transmitting elongated arc tube enclosing a predetermined volume;

(b) a light transmitting envelope spaced from and surrounding said are tube;

(c) are supporting electrodes disposed within said are tube adjacent the ends thereof;

(01) lead-in conductors connected to said electrodes and sealed through said are tube; and

(e) a discharge sustaining filling within said are tube comprising, mercury in an amount of from about 5.0 to 8.0 milligrams per cubic centimeter of arc tube volume and which when fully vaporized during normal operation of said lamp will provide a predetermined mercury density in said are tube, at least approximately 0.5 milligram per cubic centimeter of arc tube volume of thallium iodide, whereby upon full vaporization of said mercury and substantial vaporization of said thallium iodide an output in excess of 100 lumens per watt is produced, and the thermal conductivity of said are tube and said predetermined power input bearing such relationship that, when operated, the minimum temperature on said tube is at least 650 C.

7. A discharge lamp according to claim 6 wherein said amount of mercury is approximately 6.0- milligrams per cubic centimeter of arc tube volume.

8. A discharge lamp according to claim 6 wherein the distance between the ends of the are supporting electrodes remote from their adjacent arc tube ends and their adjacent are tube ends is less than 6 millimeters.

9. A discharge lamp according to claim 6 wherein a band of energy approximately 50 angstroms in Width is produced in the area of 5350 angstroms.

10. A discharge lamp according to claim 7 wherein said thallium iodide of at least approximately 0.5 milligram per cubic centimeter is 2.3 milligrams per cubic centimeter of arc tube volume.

References Cited UNITED STATES PATENTS Reiling 313-229 X Pennington 313-225 X Timmermans et a1. 313-228 X Hinman 313-184 X Rosenberg 313-184 X OTHER REFERENCES Characteristics of Mercury Vapor- Metallic Iodide Arc Lamps, Journal of the Optical So- Gilbert H. Rellrng:

ciety of America, vol.

54, N0. 1, April 1964, pp. 532-540.

ROBERT SEGAL, Primary Examiner.

U.S. Cl. X.R. 

